These are the commercial jet engines you see, like the Boeing 777, 747, Airbus A330 and A380s. Though there is a considerable reduction of the engine thrust, at these altitudes the lower drag associated with low air density means a more economical flight with a higher True Air Speed.
Crepitus in the knees may be caused by knee injuries such as meniscus tears. These are fairly common in people who play sports, jog, or run. A meniscus tear can cause crepitus as the joint moves. Chondromalacia patella is a dull ache behind the kneecap, usually caused by overuse or injury. There will be damage to the undersurface cartilage covering the kneecap.
Chondromalacia patella is abnormal softening of the cartilage of the underside the kneecap (patella). It is a cause of pain in the front of the knee (anterior knee pain). Chondromalacia patella is one of the most common causes of chronic knee pain. Chondromalacia patella results from degeneration of cartilage due to poor alignment of the kneecap (patella) as it slides over the lower end of the thighbone (femur). This process is sometimes referred to as patellofemoral syndrome.